Preparation of alkanes

Industrial sources and synthesis

Industrial production is aimed at generating large-scale volumes of hydrocarbons. As a rule, the yield consists not of pure substances, but of their mixtures (fractions).

• Petroleum processing. Petroleum serves as a practically inexhaustible source of alkanes. The main technological process here is fractional distillation (rectification). Separating petroleum into absolutely pure individual molecules is extremely difficult, so it is separated into temperature fractions.
• Coal hydrogenation. Under harsh conditions (a temperature of about $450^\circ C$ and a colossal pressure of $300\ \text{atm}$), so-called synthetic gasoline—a mixture of alkanes—is obtained from coal. However, this route is still economically inferior to petroleum refining.
• Use of synthesis gas. A mixture of carbon monoxide(II) and hydrogen ($CO + H_2$) makes it possible to synthesize various products depending on the catalyst. If nickel ($Ni$) and heating to $300^\circ C$ are used, methane is formed:

$$CO + 3H_2 \xrightarrow{Ni, 300^\circ C} CH_4 + H_2O$$ If cobalt catalysts ($Co$) are used instead, a complex mixture of alkanes and cycloalkanes is synthesized.

Laboratory methods: chain construction

In the laboratory, it is important for chemists to obtain a specific, chemically pure alkane. Name reactions are used for this purpose, allowing control over the length of the carbon skeleton.

1. Wurtz reaction (chain lengthening) The essence of the method lies in the interaction of haloalkanes with active alkali metals, most often sodium or potassium. The reaction proceeds via a free-radical mechanism:

1. The metal abstracts the halogen, forming a free radical: $R-CH_2Br + Na \rightarrow R-CH_2\bullet + NaBr$
2. Two resulting radicals recombine (join together), doubling the chain: $2R-CH_2\bullet \rightarrow R-CH_2-CH_2-R$

Important nuance: if a mixture of two different haloalkanes is taken (e.g., 1-bromopropane and 2-bromopropane), the radicals will meet randomly. As a result, a mixture of three different hydrocarbons will be formed immediately (in this example, hexane, 2,3-dimethylbutane, and 2-methylpentane).

2. Dumas reaction (chain shortening) This is a decarboxylation process. If a sodium salt of a carboxylic acid is fused with an alkali at $300^\circ C$, the carboxyl group is cleaved from the molecule. The carbon chain thus becomes one atom shorter. For example, sodium propionate yields ethane: $$C_2H_5COONa + NaOH \xrightarrow{300^\circ C} C_2H_6\uparrow + Na_2CO_3$$

3. Kolbe electrolysis Under the influence of electric current, an aqueous solution of a carboxylic acid salt also undergoes decarboxylation, but the remaining radicals couple with each other (double). The target alkane and carbon dioxide are released at the anode, while hydrogen and alkali are formed at the cathode.

Specific syntheses and reduction

In addition to manipulating the carbon skeleton, alkanes can be obtained by hydrogenating other classes of compounds.

• Preparation of methane via hydrolysis. In the laboratory, methane is conveniently obtained by the decomposition of aluminum carbide with water. Gaseous methane and a precipitate of aluminum hydroxide are formed:

$$Al_4C_3 + 12H_2O = 3CH_4\uparrow + 4Al(OH)_3\downarrow$$

• Hydrogenation of alkenes. The addition of molecular hydrogen to unsaturated hydrocarbons occurs in the presence of a nickel catalyst, converting the double bond into a single bond.
• Reduction of haloalkanes. The halogen in the molecule can be replaced by hydrogen. This is done either with hydrogen gas over a nickel catalyst or using a strong reducing agent—hydrogen iodide ($HI$)—with the simultaneous release of free iodine ($I_2$).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanism of free-radical substitution and recombination
• Cross-coupling products in the Wurtz reaction using specific examples
• Specificity of cobalt and nickel catalysts with synthesis gas
• The role of hydrogen iodide as a reducing agent in organic chemistry

Other topics in "Chapter 15. Hydrocarbons"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.